A wide variety of needleless injectors are known in the art. Examples of such injectors include those described in U.S. Pat. No. 5,599,302 issued to Lilley et al., U.S. Pat. No. 5,062,830 to Dunlap, and U.S. Pat. No. 4,790,824 to Morrow et al. In general, these and similar injectors administer medication as a fine, high velocity jet delivered under sufficient pressure to enable the jet to pass through the skin and enter the underlying tissues. These injectors typically have a nozzle assembly which has a barrel-like nozzle body having a medication holding chamber and an orifice through which a jet stream of medication is expelled from the chamber. Typically a plunger/piston actuated by an energy source, such as a coil spring, gas spring, or gas cartridge is used to expel the medicament.
Since at least the 1980s, the use of needleless injectors has become more desirable due to concerns over the spread of AIDS, hepatitis and other viral diseases caused by the possibility of accidental needle “sticks” from the conventional syringe and needle. One of the advantages associated with jet injectors is the absence of a hypodermic needle which removes apprehensions of healthcare workers and are superior in eliminating accidental disease transmission. Furthermore, given the aversion to needles possessed by some, the absence of a needle provides a psychological benefit. Even devices that utilize conventional hypodermic needles have attempted to capitalize on this psychological benefit. For example, self-injectors or auto-injectors like the ones disclosed in U.S. Pat. Nos. 4,553,962, 4,378,015 have retractable needles which are hidden until activation. Upon activation, the needle extends from the bottom of the device and penetrates the user's skin to deliver medicament. As none of these devices involves delivery of the medicament using jet injection, the medicament delivery location is limited by the length of the needle.
As the skin is a tissue composed of several layers and the injector is applied to the external surface of the outermost layer, the delivery pressure must be high enough to penetrate all layers of the skin. The layers of skin include, the epidermis, the outermost layer of skin, the dermis, and the subcutaneous region. The required delivery pressure is typically greater than approximately 4,000 p.s.i., as measured by the force of the stream of fluid divided by the cross-sectional area of the stream of fluid.
Although this pressure is readily achievable with most injectors, there are some circumstances in which delivery under a reduced pressure is desirable. For instance, certain medications which contain molecules with long protein chains can be sheared and rendered ineffective when expelled at high pressures. Reduced pressure delivery is particularly useful in intradermal applications such as vaccine, specifically DNA vaccines in which a high force energy mechanism could disrupt the molecular structure. See “Intradermal DNA Immunization by Using Jet-Injectors in Mice and Monkeys,” Vaccine, 17:628–38, February 1999. Also, operation of the needleless injector at lower pressures can allow for a larger volume of medicament to be administered. Furthermore, the lower pressure could make manufacturing an injector device less expensive. The lower pressure would also reduce adverse stresses on the device and result in a corresponding increased useable device lifetime.
Thus, there exists a need for a jet injector with an injection assisting probe to reduce the pressure at which the jet injector must eject the medicament for proper delivery.